1. Field of the Invention
The present invention relates to a calibration method for a feedback control system for a two-stage actuator.
2. Description of the Related Art
In recent years, the size and thickness of a magnetic disk drive as a kind of external storage device for a computer have increasingly been reduced, and a low power consumption has been desired. Further, high-density recording on a magnetic disk drive has also been required. For realization of high-density recording on a magnetic disk drive, it is essential to increase the number of tracks per unit length on a magnetic disk, that is, to reduce the width of each track. It is therefore necessary to position a magnetic head on a narrow track, causing the necessity of improvement in head positioning accuracy.
In a general magnetic disk drive, an actuator arm is rotatably mounted on a base, and a suspension is fixed at its base end portion to one end of the actuator arm. A slider carrying a magnetic head is mounted on a front end portion of the suspension. A coil is mounted on the other end of the actuator arm, and a magnetic circuit is fixed to the base of the magnetic disk drive. The coil and the magnetic circuit constitute a voice coil motor. By passing an electric current through the coil, the coil is forced to rotate the actuator arm.
Such a general single actuator has the following problems.
(a) In an actuator used in a general 2.5-inch or 3.5-inch magnetic disk drive, there occurs resonance due to the rigidity of an actuator arm at 10 kHz or less. It is difficult to greatly raise this resonance frequency because of various constraints including yaw angle conditions and power consumption.
(b) The resonance frequency of an actuator translation mode due to the rigidity of bearings also appears at 10 kHz or less, e.g., 4 kHz to 5 kHz. Although a preload on the bearings is changed, the rigidity does not become so large, and it is therefore difficult to raise the resonance frequency.
Thus, the resonances as described in the above paragraphs (a) and (b) occur in a general magnetic disk drive in the prior art, so that a servo band cannot be raised to a frequency higher than about 1 kHz. Accordingly, a tracking error cannot be sufficiently compressed, and it is therefore very difficult to improve a track pitch. To cope with this, there has been proposed a so-called two-stage actuator having a main actuator and a subactuator mounted on the main actuator. A voice coil motor (VCM) is used as driving means for the main actuator as in the prior art, and a piezoelectric element is used as driving means for the subactuator, thereby achieving accurate positioning of a head. For example, two piezoelectric elements are located on the opposite sides of an actuator arm, wherein a voltage is applied to the piezoelectric elements in such a direction that one of the piezoelectric elements expands and the other contracts. As a result, the head is rotated to the side where the other piezoelectric element is located.
Another type two-stage actuator employing an electrostatic actuator or an electromagnetic actuator as a subactuator has also been proposed. Each of the piezoelectric actuator and the electrostatic actuator is a kind of displacement type actuator such that if an input to the subactuator is constant, the condition of the subactuator is stable. In contrast, the electromagnetic actuator is an acceleration type actuator, the condition of which is unstable.
In general, the actuator of a magnetic disk drive is feedback-controlled. At the time of factory shipment of a magnetic disk drive, the calibration of a feedback control system is carried out to decide parameters of the control system. Further, because the environment changes in the use of the magnetic disk drive, the calibration of the feedback control system is carried out at given periods or at starting up the magnetic disk drive to thereby correct the parameters of the feedback control system to optimum values.
A calibration method for a feedback control system in the prior art will now be described with reference to FIG. 1. An actuator 2 in a prior art magnetic disk drive is a single actuator using a VCM as driving means. FIG. 1 is a block diagram of a feedback control system for the single actuator 2. The control system for the actuator 2 is a one-input one-output system having an input u(k) and an output y(k), and the parameter to be calibrated is only an equivalent gain kv. The input-displacement characteristic of the actuator 2 is approximated by kv/s2 where s is a Laplace operator. In other words, the dynamic characteristic of the actuator 2 is obtained by a double integral of input x kv.
A controller 4 is composed of a nominal controller 6 and a gain compensator 8 having a compensation gain Kcv. In the conventional calibration method, Kcv x kv is adjusted so as to coincide with a nominal equivalent gain KVN. That is, a sinusoidal disturbance d(k) generated in a disturbance generator 10 is inserted into a control input u(k) to the actuator 2, and a response (open-loop gain) to the insertion is measured to thereby execute the calibration. Alternatively, a specified acceleration is input for a given period of time without the control, and a resultant velocity or traveled distance is measured to thereby execute the calibration.
In the case that the actuator control system has one input and one output and that only one parameter is to be calibrated, the above calibration method is applicable to a single actuator. However, the above conventional calibration method is not applicable to a double (two-stage) actuator in the case that the actuator control system has two inputs and one output and that two or more parameters are to be calibrated.
It is therefore an object of the present invention to provide an effective calibration method for a feedback control system for a two-stage actuator.
In accordance with an aspect of the present invention, there is provided a calibration method for a feedback control system for a two-stage actuator having a main actuator using a voice coil motor as driving means and a subactuator mounted on the main actuator, comprising the steps of cutting off a control loop for the subactuator to fix a control input up(k) to the subactuator to 0; inserting a sinusoidal disturbance dv(k) into a control input uv(k) to the main actuator; comparing uv(k) and uv(k)+dv(k) to obtain an open-loop gain; setting the reciprocal of the open-loop gain in a gain compensator for the main actuator; adding a sinusoidal disturbance dp(k) to an input to the subactuator with the control loop for the subactuator being kept cut off; comparing the sinusoidal disturbance dp(k) with an output y(k) from the two-stage actuator to obtain a disturbance input-displacement gain; dividing a design nominal value by the disturbance input-displacement gain to obtain a compensation gain; and setting the compensation gain in a gain compensator for the subactuator.
In accordance with another aspect of the present invention, there is provided a calibration method for a feedback control system for a two-stage actuator having a main actuator using a voice coil motor as driving means and a subactuator mounted on the main actuator, comprising the steps of inserting a first sinusoidal disturbance dv(k) into a control input uv(k) to the main actuator; comparing the first sinusoidal disturbance dv(k) with an output y(k) from the two-stage actuator to obtain a first gain of disturbance input-positional error; dividing a first design nominal value by the first gain of disturbance input-positional error to obtain a first compensation gain; setting the first compensation gain in a gain compensator for the main actuator; inserting a second sinusoidal disturbance dp(k) into a control input up(k) to the subactuator; comparing the second sinusoidal disturbance dp(k) with the output y(k) from the two-stage actuator to obtain a second gain of disturbance input-positional error; dividing a second design nominal value by the second gain of disturbance input-positional error to obtain a second compensation gain; and setting the second compensation gain in a gain compensator for the subactuator.
Preferably, the step of lowering a control band of the control system as a whole by a given band is inserted before the step of inserting the first sinusoidal disturbance into the control input to the main actuator, whereby the frequency characteristic of an error transfer function becomes flat at 0 dB in a wide range. Since the gain of the error transfer function becomes 1 (0 dB) in a wide frequency band, the first and second compensation gains can be easily obtained.
Further, the step of obtaining the first compensation gain for the main actuator and the step of obtaining the second compensation gain for the subactuator may be simultaneously carried out by using sinusoidal disturbances having different frequencies, thereby reducing the calibration time. In this case, the frequency of the sinusoidal disturbance for the subactuator, that is, the frequency of the second sinusoidal disturbance is set higher. The frequency of the second sinusoidal disturbance is required to be at least 4/3 times the frequency of the first sinusoidal disturbance.
In accordance with a further aspect of the present invention, there is provided a calibration method for a feedback control system for a two-stage actuator having a main actuator using a voice coil motor as driving means, a plurality of displacement type subactuators mounted on the main actuator, a plurality of heads respectively mounted on the displacement type subactuators, first and second head ICs, and first and second demodulators, comprising the steps of cutting off a control loop for each of the subactuators to fix a control input up(k) to each subactuator to 0; inserting a sinusoidal disturbance dv(k) into a control input uv(k) to the main actuator; comparing uv(k) and uv(k)+dv(k) to obtain an open-loop gain; setting the reciprocal of the open-loop gain in a gain compensator for the main actuator; feedback controlling the main actuator and one of the subactuators to move one of the heads onto a track; changing an input to another one of the subactuators; demodulating the position of the head mounted on the another subactuator to obtain an input-displacement gain; dividing a design nominal value by the input-displacement gain to obtain a compensation gain; and setting the compensation gain in a gain compensator for the another subactuator.
In the case of using an accelerator type subactuator such as an electromagnetic actuator in place of each accelerator type subactuator, the step of changing an input to another one of the subactuators is replaced by the step of adding a sinusoidal input to another one of the subactuators. The other steps are similar to those in the case of using the displacement type subactuators mentioned above.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.